1. Field of the Invention
The present invention relates to a radiation image capturing apparatus including a radiation source and a radiation conversion panel for detecting radiation emitted from the radiation source, which has passed through a subject, and converting the detected radiation into radiation image information.
2. Description of the Related Art
Radiation image capturing apparatuses having a radiation conversion panel for detecting radiation emitted from a radiation source that has passed through a subject and converting the detected radiation into radiation image information are disclosed in Japanese Laid-Open Patent Publication No. 09-311187, Japanese Laid-Open Patent Publication No. 07-265286, Japanese Laid-Open Patent Publication No. 2003-151795, and Japanese Laid-Open Patent Publication No. 2005-192856, for example.
Specifically, Japanese Laid-Open Patent Publication No. 09-311187 discloses a nuclear medicine image diagnostic apparatus for displaying a life-sized radioactive material distribution image of a subject in the same position as the subject. In the nuclear medicine image diagnostic apparatus, radiation applied from a collimator mounted on a rear surface of the apparatus is detected by a semiconductor detector, and positions where radiation is applied are successively calculated and stored by a processor, and then displayed on a liquid crystal display panel mounted on a front surface of the apparatus.
Japanese Laid-Open Patent Publication No. 07-265286 discloses an X-ray diagnostic apparatus, which also is capable of outputting a life-sized image. The disclosed X-ray diagnostic apparatus comprises an X-ray tube, a top plate, an image intensifier, a TV camera for capturing an optical image from the image intensifier, and an output device for outputting the image captured by the TV camera. The X-ray diagnostic apparatus also includes an SID measuring unit for measuring a distance (SID) from the X-ray tube to the X-ray irradiated surface of the image intensifier, and an FSD measuring unit for determining a distance (FSD) from the center of a subject to the X-ray irradiated surface of the image intensifier. Based on the SID and the FSD, the X-ray diagnostic apparatus determines an enlargement ratio and outputs the captured image in full scale. The FSD measuring unit estimates the FSD based on detected values from a sensor for detecting the vertical position of the top plate and a sensor for detecting the vertical position of the image intensifier, and the small distance between the subject and the top plate.
Japanese Laid-Open Patent Publication No. 2003-151795 discloses an X-ray image capturing apparatus for easily generating quasi-parallel X-rays to obtain a life-sized image of a subject. The X-ray image capturing apparatus operates as follows: X-rays are output through a window. After an X-ray radiation field is limited by a diaphragm, the X-rays are applied to a flat polycapillary plate, which produces parallel X-rays. The parallel X-rays are applied to the subject to generate an image of the subject on an imaging plate. Because parallel X-rays are used, the generated image produces a life-sized image of the subject.
Japanese Laid-Open Patent Publication No. 2005-192856 discloses that the distance from the front surface of an X-ray image intensifier that makes up an X-ray detector to the affected area of a subject is set to a desired distance. The distance from the position of an image-capturing surface to the reference position of the subject also is entered. The publication further discloses a life-sized image data correction technique, based on calculating a magnification ratio using the distance between the front surface of the X-ray detector and the affected area of the subject.
The apparatuses disclosed in Japanese Laid-Open Patent Publication No. 09-311187 and Japanese Laid-Open Patent Publication No. 2003-151795 are disadvantageous in that they are large in size and costly to manufacture, due to the fact that a collimator means is required for optically converting the radiated X-rays into parallel X-rays.
According to the X-ray diagnostic apparatus disclosed in Japanese Laid-Open Patent Publication No. 07-265286, the FSD is determined based on the top plate as a reference. However, the reference provided by the top plate tends to fluctuate, since the top plate may warp under the weight of the subject that is supported by the top plate. In addition, a preset fixed value is used as the distance between the central position of the subject and the top plate. However, the distance between the central position of the subject and the top plate may change greatly, depending on the physique of the subject. Consequently, the X-ray diagnostic apparatus disclosed in Japanese Laid-Open Patent Publication No. 07-265286 fails to convert the acquired image into a life-sized image accurately.
According to Japanese Laid-Open Patent Publication No. 2005-192856, the distance from the position of the image-capturing surface to the reference position of the subject can be set to an arbitrary distance. However, no consideration is given to facilitating acquisition of life-sized image information from the radiation conversion panel at an arbitrary position, based on the radiation image information that has already been acquired, for thereby shortening medical procedures that are performed using the radiation image capturing apparatus. Furthermore, although the image corresponding to the reference position is corrected to provide a life-sized image, images of other regions of the subject are of sizes that are different from the actual size of the subject. If the operator is not certain as to whether the image displayed on the display monitor has been corrected to provide a life size or not, then the operator possibly could misidentify the size and position of the region of the subject in question.